The images on your phone screen are made up of hundreds of thousands of tiny dots of colour called pixels. Pixels can be any size but smaller pixels give things more definition. For a screen to qualify as High Definition (HD) it has to have a minimum number of pixels to make sure they’re tiny. That means they measure 1280 pixels by 720 pixels, so are called 720p.

But this isn’t the sharpest resolution we have in our phones right now. In 2013 we started to get Full HD screens with 1080p resolutions that measure 1920×1080 pixels, like the one on the Pixel, a phone by Google. And in 2014 we got our first Quad HD screens (which measure 2560x1440 pixels) now seen on the likes of the Samsung Galaxy S7 and the Pixel XL.

The pixels on these screens are so small that you can only see each block of colour if you really look for them. Now, we have the world's first 4K smartphone screen on the Sony Xperia Z5 Premium - that's unbelieavably sharp.

4K screens – 4096 × 2160 pixels

4K is the sharpest of the new resolutions that everyone’s talking about. The new Sony Xperia Z5 Premium (and likely all future 4K phones) doesn't use true 4K, though. Videos in 4K have odd proportions that would mean they would be squashed out of shape on a TV or smartphone. Instead, the new Sony uses Ultra HD, which is a slightly different standard.

Ultra HD screens – 3840 × 2160 pixels

Ultra HD, or 2160p, has four times as many pixels as the 1080p Full HD standard that we’re used to. That’s why its technical name is Quad Full HD (QFHD). Some phones can already record video in this incredibly sharp standard. The Samsung Galaxy Note 3 was the first phone to shoot Ultra HD video. But it was marketed as 4K as there’s such a small difference between true 4K and Ultra HD.

Quad HD screens – 2560 × 1440 pixels

Quad HD (QHD) is the resolution that really took off year in 2015. It gets its name from being four times sharper than 720p HD. Be careful not to confuse Quad HD with qHD (using a lowercase Q), as this stands for quater HD - with a resoltuion measuring 960 x 540 pixels.

2K screens – 2048 × 1152 pixels

2K has been the minimum standard for blockbuster movies for about 10 years, but we’re not expecting it to ever make it big with smartphones. The reason is that it only gives a small boost in sharpness compared to 1080p Full HD, so most major smartphone makers will jump straight to QHD.

Retina Display

Retina display is a brand name for Apple’s screens, so doesn’t have a fixed resolution. But according to Apple, a Retina display is defined by pixel size. The pixels should be so small that you can’t see each one. So when you get a big Retina display like on the iPad Air, the resolution goes up so you have enough of the tiny pixels to fill the screen.